The subject matter disclosed herein relates to turbine systems, and more particularly to a compressor start bleed system, as well as a method of controlling the compressor start bleed system.
At low speeds, an axial compressor tends to load the front end of the compressor, while the back end is very lightly loaded. This characteristic can result in an inoperable condition with the front of the compressor in a surge condition, which can result in mechanical stress, loss of air flow and the failure of the turbine system to start. To address this characteristic, a start bleed system is employed. The start bleed system extracts flow from one or more compressor stages, which unloads the front of the compressor and moves it away from the surge boundary. This will also increase the loading on the back of the compressor, but given the light loading, there is typically margin to increase the loading without risking surge. Start bleed systems typically employ an on/off control, and the target flow is selected to balance the operation of the various regions of the compressor. Depending on the location of the start bleeds within the compressor, and the characteristics of the compressor, the start bleed extraction from the compressor may decrease, or increase the compressor discharge flow to the rest of the gas turbine system.
In addition to the compressor limitations, there are also limitations on combustor operability which tend to require a limited range of fuel flow relative to air flow, and other system limitations such as exhaust temperature, turbine acceleration, etc. The typical gas turbine design sets a start bleed flow, and then based on the compressor performance with start bleeds on, a control strategy is developed to meet the system boundaries. The current turbine systems tend to target higher performance, and the resulting operable space for the system is becoming smaller. Therefore, it is becoming increasingly more challenging to meet all the system requirements with the typical system design.